mercy was one of his success factors that he properly used for the cause of his call. allah may he be glorified say:
"and by the mercy of allah, you dealt with them gently. and had you been severe and harsh hearted they would have broken away from about you;" (quran 3:159)
in a simple way, the noble ayah means: if it was not for your mercy o muhammad, they would have turned away rejecting your call..
his mercy, blessing and peace be upon him, is actually an evidence to his genius and insightfulness. in other words, his mercy served as another dimension to his genius and insightfulness. matter of fact, this mercy stood among his success factors that he tapped on for winning hearts. meanwhile, he even turned it into means to open up black hearts, and move dumb feelings. however, this should not be taken as putting on an artificial cloak of mercy and using it as a vehicle to attain illegal aims. but it is rather part and parcel of his nature and instinct just as allah created him.
adopting a merciful approach sincerely for the sake of allah stands on one side with that type of mercy practiced to secure lowly personal gains right on the opposite side. there is no comparison - non what so ever - between adopting mercy for a noble message and that mercy seeking vanishing leadership.
mercy in the hands of the messenger